The present invention relates to an optical element molded from a plastic material by using an optical insert member, a method of molding this optical element, and a laser scanning optical system using the optical element. Recently, optical elements such as lenses are manufactured by using plastic materials, because aspherical shapes can be obtained easily and plastic materials are light in weight. In addition, most optical elements consisting of plastic materials are manufactured by injection molding or molding methods, such as injection compression molding, classified into the category of injection molding. The consequent mass-productivity leads to the advantage that these optical elements are inexpensive compared to those made from glass. Conventionally, such an optical element consisting of a plastic material is generally molded by using an optical insert member 10, as shown in FIG. 1 and having a surface shape which is the exact reverse of a lens surface shape optically designed. This optical insert member 10 is constituted by optical inserts 12 and 13 forming a cavity 24, and an optical insert set 11. Surfaces 14 and 15 of the optical inserts are finished into mirror surfaces.
A molten resin is injected from a gate 21 into the cavity 24 defined by the optical inserts 12 and 13, and a sufficiently high pressure is applied. Consequently, the surface shapes of the optical inserts 12 and 13 are transferred as the optical function surfaces of an optical element as the molded product.
Molding an optical element consisting of plastics as described above, however, introduces the problem that the surface shape accuracy of the resulting optical element is reduced due to the shrinkage upon molding of the plastic material.
Assuming that a lens is molded by using plastics as a material, for example, the molded lens is smaller than the cavity 24 formed by the optical inserts 12 and 13. In addition, the optical function surfaces deform from the surfaces of the optical inserts due to the molding shrinkage.
More specifically, in the case of a lens shown in FIG. 2, this deformation is caused by shape errors in the direction of thickness and in the direction of the generatrix (longitudinal direction), and by the resulting generatrix inclination, curvature error, and radius error in the direction of the directrix (lateral direction). If these errors fall outside the range of design tolerances, this molded product cannot be used as a final product.
If, however, the displacement from the surfaces of the optical inserts occurring during molding is stable and does not vary largely independent of the molding date and the molding environment, it is only necessary to correct this error in advance by the shape of the surfaces 14 and 15 of the optical insert. Consequently, the shape of the molded product can fall within the range of design tolerances, and so the molded product can be used as a final product.
Several methods are known as the method of forming an optical insert by taking into account the shrinkage or the deformation taking place during molding. In the case of lenses whose required accuracy is low, an optical insert is processed as follows. That is, if an optical function surface with R=20 is necessary, the radius of curvature of the surface of the optical insert is multiplied by an amount of molding shrinkage (in this case, 1.003 to 1.006), as 20.times.1.003=20.06 to 20.times.1.006=20.12, assuming that the molding shrinkage is an even shrinkage symmetrical about the optical axis.
In the case of lenses whose required accuracy is relatively high, on the other hand, the difference in shape of the optical function surface of a molded lens from the surface shape of an optical insert, i.e., the shrinkage deformation caused by molding is approximated by a polynomial of degrees two, four, six, . . . , and the coefficients of the degrees two, four, six, . . . of this polynomial are subtracted from the coefficients of the degrees two, four, six, . . . of a polynomial which represents the initial desired optical design shape, thereby making a polynomial representing a new surface shape. An optical insert is remanufactured on the basis of this polynomial.
In either of the above conventional methods, however, the entire lens surface is approximated by one molding shrinkage factor symmetrical about the optical axis or by one polynomial. As a result, the following problems arise.
1. In molding particularly an elongated lens or a large-diameter lens, the degree by which a resin is cooled by an optical insert in a portion closer to a gate, as a resin injection port, is different from that in a portion farther from the gate. That is, the viscosity of a resin flowing to the portion closer to the gate is low since the resin is not cooled very much, while the viscosity of a resin flowing to the portion farther from the gate is high since the resin is cooled well. Consequently, the shrinkage factor of the resin in the portion closer to the gate is different from that in the portion farther from the gate; the shrinkage factor in the portion farther from the gate is larger. The result is that the surface shape of an optical insert is not satisfactorily transferred to the resin in the portion farther from the gate, leading to degradation in surface accuracy.
2. To increase the surface accuracy in the portion farther from the gate, on the other hand, a high pressure must be applied. Consequently, when the optical insert set is opened, release deformation is caused by an increase in elastic deformation or in release resistance resulting from the over packing of a resin. This degrades the surface accuracy in the portion closer to the gate.
3. Furthermore, in molding of an optical element having a complicated shape, a partial surface defect (=degradation of the surface accuracy) occurs due to uneven cooling and shrinkage on the optical surface. This makes it difficult to obtain a uniform surface accuracy.
In molding of plastic optical elements, it is impossible to form the gate in a portion of an optical axis 36 (a portion along the central line of a lens) since the optical axis is necessary as an effective portion. That is, as illustrated in FIG. 2, the gate is positioned at one end of a lens. In many instances, therefore, no shrinkage deformation symmetrical about the optical axis 36 (central line) takes place under the influence of the flow of a resin. However, there is no conventional means of processing an optical insert by taking these local shrinkage and deformation into consideration. That is, there is no means of correcting the local shrinkage and deformation of an optical element by using the optical insert.
Conventionally, therefore, the following special molding methods are used or the molding conditions are improved in order to minimize the shrinkage deformation from the surface shape of an optical insert and to obtain an even shape free from a habit.
1. A so-called injection compression molding method is used in which a pressure is not applied into a cavity from a single gate, but compression is performed by the entire optical insert.
2. A resin is injected with the mold temperature of the optical insert set raised to be higher than the glass transition point of the resin, and then cooled to extract the molded product.
Either of the above two methods, however, requires a large-scale apparatus or is unsuitable for mass-production because the molding cycle is long. In either case, the manufacturing cost is increased. In addition, even if the above special methods are used, it is not necessarily possible to obtain a desired uniform shape accuracy throughout the entire optical function surface depending on the shape or the like of an optical element. Also, even if polynomial approximation is possible, the degree of the polynomial increases to cause large oscillations around both the ends of data, or to make the data susceptible to errors in arithmetic operations performed by a computer. All these factors make it difficult to increase the accuracy of plastic optical elements.